THE ALOHA SYSTEM—Another alternative for computer communications* by NORMAN ABRAMSON University of Hawaii Honolulu, Hawaii INTRODUCTION WIRE COMMUNICATIONS AND RADIO COMMUNICATIONS FOR COMPUTERS In September 1968 the University of Hawaii began work on a research program to investigate the use of radio communications for computer-computer and console-computer links. In this report we describe a remote-access computer system—THE ALOHA SYS-TEM—under development as part of that research program1 and discuss some advantages of radio com-munications over conventional wire communications for interactive users of a large computer system. Al-though THE ALOHA SYSTEM research program is composed of a large number of research projects, in this report we shall be concerned primarily with a novel form of random-access radio communications developed for use within THE ALOHA SYSTEM. The University of Hawaii is composed of a main campus in Manoa Valley near Honolulu, a four year college in Hilo, Hawaii and five two year community colleges on the islands of Oahu, Kauai, Maui and Hawaii. In addition, the University operates a number of research institutes with operating units distributed throughout the state within a radius of 200 miles from Honolulu. The computing center on the main campus operates an IBM 360/65 with a 750 K byte core memory and several of the other University units operate smaller machines. A time-sharing system UHTSS/2, written in XPL and developed as a joint project of the Univer-sity Computer Center and THE ALOHA SYSTEM under the direction of W. W. Peterson is now operating. THE ALOHA SYSTEM plans to link interactive com-puter users and remote-access input-output devices away from the main campus to the central computer via UHF radio communication channels. * THE ALOHA SYSTEM is supported by the Office of Aero-space Research (SRMA) under Contract Number F44620-69-C-0030, a Project THEMIS award. At the present time conventional methods of remote access to a large information processing system are limited to wire communications—either leased lines or dial-up telephone connections. In some situations these alternatives provide adequate capabilities for the de-signer of a computer-communication system. In other situations however the limitations imposed by wire communications restrict the usefulness of remote access computing.2 The goal of THE ALOHA SYSTEM is to provide another alternative for the system designer and to determine those situations where radio com-munications are preferable to conventional wire communications. The reasons for widespread use of wire communica-tions in present day computer-communication systems are not hard to see. Where dial-up telephones and leased lines are available they can provide inexpensive and moderately reliable communications using an existing and well developed technology.3,4 For short distances the expense of wire communications for most applica-tions is not great. Nevertheless there are a number of characteristics of wire communications which can serve as drawbacks in the transmission of binary data. The connect time for dial-up lines may be too long for some applications; data rates on such lines are fixed and limited. Leased lines may sometimes be obtained at a variety of data rates, but at a premium cost. For communication links over large distances (say 100 miles) the cost of com-munication for an interactive user on an alphanumeric console can easily exceed the cost of computation.5 Finally we note that in many parts of the world a reliable high quality wire communication network is not available and the use of radio communications for data transmission is the only alternative. There are of course some fundamental differences 281282 Fall Joint Computer Conference, 1970 between the data transmitted in an interactive time-shared computer system and the voice signals for which the telephone system is designed.6 First among these differences is the burst nature of the communication from a user console to the computer and back. The typical 110 baud console may be used at an average data rate of from 1 to 10 baud over a dial-up or leased line capable of transmitting at a rate of from 2400 to 9600 baud. Data transmitted in a time-shared com-puter system comes in a sequence of bursts with ex-tremely long periods of silence between the bursts. If several interactive consoles can be placed in close proximity to each other, multiplexing and data con-centration may alleviate this difficulty to some extent. When efficient data concentration is not feasible how-ever the user of an alphanumeric console connected by a leased line may find his major costs arising from communication rather than computation, while the communication system used is operated at less than 1 percent of its capacity. Another fundamental difference between the require-ments of data communications for time-shared systems and voice communications is the asymmetric nature of the communications required for the user of inter-active alphanumeric consoles. Statistical analyses of existing systems indicate that the average amount of data transmitted from the central system to the user may be as much as an order of magnitude greater than the amount transmitted from the user to the central system.6 For wire communications it is usually not possible to arrange for different capacity channels in the two directions so that this asymmetry is a further factor in the inefficient use of the wire communication channel. The reliability requirements of data communications constitute another difference between data communica-tion for computers and voice communication. In addi-tion to errors in binary data caused by random and burst noise, the dial-up channel can produce connection problems—e.g., busy signals, wrong numbers and dis-connects. Meaningful statistics on both of these prob-lems are difficult to obtain and vary from location to location, but there is little doubt that in many loca-tions the reliability of wire communications is well be-low that of the remainder of the computer-communica-tion system. Furthermore, since wire communications are usually obtained from the common carriers this portion of the overall computer-communication system is the only portion not under direct control of the system designer. THE ALOHA SYSTEM The central computer of THE ALOHA SYSTEM (an IBM 360/65) is linked to the radio communication CENTRAL COMPUTER IBM 360/63 <h HP 2115 A TRANSMIT DATA